First sets with efficiency diode & LOPT

[Panrock]

Somebody asked me this the other day and I had to admit I didn't know.

What was the first British set that used an Efficiency Diode?

And while we're at it, what was the first British set with EHT generated by the line output transformer? Was it the Pye B18T? (The 1939 German Telefunken E1 doesn't count).

Thanks.

Steve

[ukcol]

According to Jon the D/B18T was the first to us a boost diode.

https://www.vintage-radio.net/forum/...ad.php?t=11592

[Panrock]

Thank you! That link completely answers my questions.

Steve

[Philips210]

Hi.

Slightly OT I know, but talking of efficiency diodes, I think the GEC hybrid colour set must have been the only one to be initially produced with a solid state diode. It looked odd to see the PL509 line output valve minus it's PY500A boost diode companion. Later sets had the PY500A fitted.

Regards
Symon.

[Oliver35]

Hugo Holden (I think) wrote a very detailed article on the development of the efficiency (or damper) diode, very informative. He mentions some landmark sets, some British, some American, but which ones and in what connection I now forget.

[FERNSEH]

I'd say the 1946 RCA model 630TS was possibly the first set to use a damper and reclaim energy diode: http://www.earlytelevision.org/rca_630.html
Read the technical description of this set. The model 621TS was the 7" CRT version of the 630.
As for UK made sets 1948 appears to the year when many set makers employed an efficiency diode. The Ekco TS88 employs a PY31 which supplied reclaimed energy to the line output valve. The McMichael model 909 used a Mullard EZ35/6X5 as a damper diode. The circuit of this set is drawn in a rather strange manner so further study will be required to determine if this is an energy reclaim circuit. But we have to differentiate between a damper diode and a true energy retrieval diode. The subject of boost diodes has been discussed before on this forum. In the early days of magnetic scanning the "ring" produced by the rapid collapsing scanning stroke was simply damped by a resistor across the scanning coils, very wasteful of energy. Next came along a frequency selective capacitor in series with the damping resistor, at least the only higher frequency flyback component of the scanning waveform was damped. By the addition of a diode only one half of the flyback is damped so why not charge a capacitor from the flyback pulse and use the energy to boost the power supply to the line output valve?
This is what is done in the 1949 Murphy V120C. The U403 efficiency diode supplies a negative voltage to the cathode of the PEN46 line output valve.
The pre-war Baird television models T18 and T20 employed a diode which is connected in parallel with the line output valve. The diode conducted at the beginning of the scanning stroke. Thirty years later transistor line output stages worked in a similar manner

DFWB.

[FERNSEH]

Originally Posted by Panrock

And while we're at it, what was the first British set with EHT generated by the line output transformer? Was it the Pye B18T? (The 1939 German Telefunken E1 doesn't count).

How about the Philips 563A? This set employed a pair of HVR2 rectifier diodes in a voltage doubler circuit. The flyback pulse came direct from the anode of the line output valve.

DFWB.

[DangerMan]

There was a "Patent Pool" in operation at the time, which may go some way to explain why a rash of sets suddenly surfaced using the B18T's line output/boost diode / EHT arrangement.

Certainly, the TS88 and Ferguson 941 use the negative boost line topology of the Pye, which was attributed to a Dr Lax* who had been with Telefunken pre-war and remembered its principle (ref: "Setmakers by Geddes & Bussey)

The circuit is a clever one for the day in that it required no great heater-cathode insulation (because the boost diode's cathode is connected directly to chassis), yet it still required less overall energy input than a "conventional" circuit (ie. one without an efficiency diode).
Plus.. as the newly-found boost voltage is effectively added to the HT voltage feed to the line output stage (LOP valve's cathode is connected to the -ve boost line) there is now sufficient voltage available to do the job without the need for an HT line voltage above that of rectified mains. By also using the flyback pulse for EHT at the same time no mains transformer was needed at all.

It's a good job the EY51 was about though!

* The same Dr Lax who led the team that developed the PAM710 transistor radio.

[Heatercathodeshort]

Dr Lax. Another uncrowned hero. J.

[FERNSEH]

The damper diode circuit used in the 1949 McMichael models 909 and 912. The EZ35/6X5 diode has been redrawn as a semiconductor device.
This circuit arrangement does not contribute any reclaimed energy back to the line output stage.

DFWB.

[kalee20]

No, though at least some of the flyback energy is reclaimed as EHT.

[FERNSEH]

The line timebase in the 1949 Sobell model T90.
In this circuit V19 is a true reclaim diode, a Mazda U403. A highly insulated 40 volt heater winding on the mains transformer is needed for this valve which was originally designed an HT rectifier valve in AC/DC radio receivers. The diode conducts on the first positive going "ring", and the reclaimed energy is supplied back to the line output stage.

DFWB.

[brianc]

ISTR that Alan Blumlein had something to do with the Boost Diode! I may be wrong and I had a quick look at the Black Book under "Picture Monitor" but could find no clear reference to a boost diode. There are two diodes used in the line scan circuitry, one to clear ringing at 50kHz and the other, 500kHz. I could not decide whether the energy "saved" was used in the line scanning process! They also used 702s as picture monitors at AP.

[FERNSEH]

Hi Brian,
the two diodes in the line output stage perform damper functions, V8 damps the oscillation produced by the inductance of the primary of the line output transformer and the lumped capacitances which work out to be 0.0001mfd. The text in the Black Book explains that as a function of L X di/dt an oscillation of 50,000c/s with an amplitude of 3000volts is present at the anode of the line output valve V7. The positive going cycle is needed for the flyback but the negative going cycles must be suppressed and V8 performs that function.
Diode V9 damps the oscillation in the scanning coils caused by the rapid flyback time. The text goes on to inform us that without the damper diode the oscillation after the flyback stroke will introduce the effect of velocity modulation on the picture.

DFWB.